viscometer

Deepak Sharma, Mankaran Singh, Dinesh Kumar, Gurmeet Singh

M.Pharmacy

PHARMACEUTICSCT Institute of Pharmaceutical sciences

Jalandhar, INDIAAICTE , PCI approved

Single point viscometer:-Equipment work at a single rate of shear.Capillary viscometer(Ostwald viscometer)Falling sphere viscometer

Application:- Newtonian fluids

Multipoint viscometer:-Equipment work at a several rate of shear.Cup and bob viscometerCone and plate viscometer

Application:- Non-Newtonian, Newtonian fluids

CAPILLARY VISCOMETER(Ostwald viscometer)

Principle: When a liquid flows by gravity

,the time required for the liquid to pass

b/w two marks through a vertical capillary

tube is determined. The time of flow of the

liquid under test is compared with the time

required for a liquid of known viscosity.

The viscosity of unknown liquid can be determined using following

equation.

n1 = p1t1 n2

p2t2

p1 -density of unknown liquid

p2 - density of known liquid

t1 -time of flow of unknown liquid

t2 -time of flow of known liquid

n2 –viscosity of known liquid

Application:-

Used for quality control purposes in the formulation and evaluation of

pharmaceutical dispersion systems such as colloids, dilute suspensions,

emulsions etc.

FALLING SPHERE VISCOMETER

Principle:-

• Apparatus consists of a glass tube positioned vertically. A

constant temperature jacket with provision for water circulation is

arranged around the glass tube.

•The test liquid is placed in the glass chamber. A glass or steel ball

is dropped into the liquid and allowed to reach equilibrium with the

temperature of the outer jacket.

•The tube with the jacket is then inverted, which places the ball at

the top of the inner glass tube. The time taken for the ball to fall b/w

two mark is accurately measured

•This process is repeated several times to obtain concurrent results.

The viscosity of a Newtonian liquid is calculated from

n1=t(Sb-Sf)B

t is the time interval in seconds for the ball to fall b/w the two

points, Sb and Sf are the specific gravity of the ball and fluid under

examination

CUP AND BOB VISCOMETERThis a multipoint viscometer and belongs to the category of rotational viscometers

Principle:-•The sample is placed in the cup and the bob is placed in cup upto an appr. height

•The sample is accommodated b/w the gap of cup and bob

•Now, neither the cup or bob is made to rotate and the torque resulting from the viscous drag is measured by a spring or sensor device in the derive of the bob

•The number of revolutions and the torque represent the rate of shear and shearing stress, respectively.

•The following equation is used to calculate the apparent viscosity of pseudoplastic system.

n= Kv w

v

Where w= weight placed on the hanger, shearing stress

v= rpm, shear rate

n= apparent viscosity of the fluid

Kv =constant for the instrument

CONE AND PLATE VISCOMETER

It possesses several advantages:-

The rate of shear is constant throughout the entire sample being

sheared.

The sample required is small 0.1 to 0.2 ml.

Cleaning and filling is easy.

Less time is required for temperature equilibrium.

Principle:

•The cone is driven by a variable-speed motor and the sample is

sheared in the narrow gap b/w the stationary plate and the rotating

cone.

•The rate of shear in rpm is increased and decreased by a selector

dial and the viscous traction or torque (shearing stress) produced on

the cone is read on the indicator scale.

•A plot of rpm (rate of shear) v/s scale reading (shearing stress) may

thus be constructed in the usual manner.

•The viscosity is estimated by equation

n=C T

v

Where C is the instrument constant

T is the torque

v is the speed of the cone

BROOKFIELD VISCOMETER

•The Brookfield Viscometer is an instrument used for measuring the

viscosity of plastisols and other liquids of a thixotropic nature.

• The instrument measures the shearing stress on a spindle rotating at

a definite, constant speed while immersed in the sample. The degree

of spindle lag is indicated on a rotating dial.

•This reading multiplied by a conversion factor based on spindle size

and rotational speed, gives a value for viscosity in centipoise.

•By taking measurements at different rotational speeds, an indication

of the degree of thixotropy of the sample is obtained.